Requirements

• When physical length is determined from electrical length, the physical length of the link calculated using the pair with the shortest electrical delay shall be reported and used for making the pass or fail decision. 
• The maximum physical length of the permanent link shall be 90 m (290 ft) (test equipment cords are excluded from the permanent link model). The maximum physical length of the channel shall be 100 m (328 ft) (including equipment cords and patch cords). 
• The Pass or Fail criteria is based on the maximum length allowed for the channel or permanent link given in figures 11-1 and 11-2 plus the nominal velocity of propagation (NVP) uncertainty of 10 percent.

NOTE - Calibration of NVP is critical to the accuracy of length measurements.

Recommendations

• The physical length of the permanent link/channel may be determined by physically measuring the length(s) of the cable(s), determined from the length markings on the cable(s), when present, or estimated from the electrical length measurement. 
• The electrical length is derived from the propagation delay of signals and depends on the construction and material properties of the cable.